1. Field of the Invention
This invention relates to volatile dispensing devices that employ active means to dispense volatiles from a volatile-impregnated substrate. More particularly, the invention relates to improved volatile impregnated substrates, such as wicks and mats, that can be used in a volatile dispensing device that employs active means to dispense.
2. Description of the Related Art
The following definitions apply herein: An “effective amount” or an “effective delivery rate” and the like term is defined to mean that amount or rate sufficient to achieve the intended effect. “Active means” is defined as heat, moving air, or other means of imparting energy to a volatile material to be dispensed, and a volatile dispensing device that employs active means to dispense volatiles will be referred to as “actively dispensing” the volatile material. To be effective, insect control volatile active ingredients have to be delivered at no less than an “insect controlling rate,” defined herein as that rate sufficient to repel or otherwise control target insects within the space to be protected.
Devices for dispensing thermally or otherwise volatilizable materials into the atmosphere are well known. Such volatilizable materials may be air scents (e.g. fragrances), pest control materials (e.g., insecticides), allergen control ingredients, disinfectants, and the like.
In one type of volatile material dispensing device, a heat source is used to promote the wicking action and release of a volatile material from a wick, one end of which is immersed in a volatile liquid contained in a reservoir. One version of this type of device is plugged into an electrical wall outlet to supply power to a heating coil within a housing. The generated heat raises the temperature of a material contained in the wick and volatilizes the material. Convection air currents dispense the volatilized material into the room. Examples of this type of device and related devices can be found in U.S. Pat. Nos. 6,361,752; 5,647,053; 5,290,546; 5,222,186; 5,095,647; 5,038,394; and 4,663,315. The disclosure of these patents and all other publications referred to herein are incorporated herein by reference as if fully set forth.
In another type of volatile material dispensing device, a stiff, porous mat (commonly made of a pressed, cellulosic, fibrous material) is impregnated with a volatile material, or a volatile material is placed in a pan-like metal structure. These mats and pans are then placed on heaters to cause the volatile material to vaporize into the atmosphere. One type of heater used for this purpose is sold by S.C. Johnson & Son, Inc. under the trademark “FUYI VAPE”. Examples of this type of device and related devices can be found in U.S. Pat. Nos. 6,446,384; 6,309,986; 6,031,967; 5,945,094; and 4,391,781.
In yet another type of volatile material dispensing device, a stiff, porous mat is impregnated with a volatile material, and the mat is held within the device such that a flow of hot gases from a fuel burner passes over the mat and causes the volatile material to vaporize into the atmosphere. An example of this type of device can be found in PCT International Patent Publication No. WO 00/78135.
In yet another type of volatile material dispensing device, moving air is directed against a substrate or through a reticulated or otherwise airflow-transmitting substrate to thereby volatilize volatile material with the substrate has been impregnated. Examples of this type of device can be found in U.S. Pat. No. 5,547,616 and in PCT International Patent Publication No. WO 01/02025.
While all of these devices provide satisfactory results under certain circumstances, there are drawbacks to each type of device.
In devices that use a heat source to promote the release of a volatile material from a wick immersed in a volatile liquid composed of solvents and dissolved active ingredients, presently available wicking materials limit the performance of the wick of the device. For instance, when using porous wicks made of felt or fabric in such a device, there is a tendency as the wick is heated for the solvents to quickly volatilize off, thereby leaving high boiling point materials behind in the wick. The high boiling point materials remaining in the wick can cause clogging of the wick. Other wick materials include ceramics and compressed sawdust. However, these wicks experience the same sort of clogging when used in a device that uses a heat source to promote the release of a volatile material from a wick immersed in a volatile liquid. As a result, it is difficult to maintain stable evaporation of the volatile liquid for extended time periods. While uniform release of volatiles has been reported in U.S. Pat. No. 4,286,754 for non-heated wicks, the problem of non-linear release in heated wicks remains.
Certain volatile materials, such as some insecticide formulations, include materials that either are or become non-volatile when the liquid is heated in a wick. These non-volatile materials can cause clogging of the wick, which leads to a less uniform release rate of the volatile during use over time and to high residual levels of non-volatile materials in an expended wick. For example, pyrethrum insecticides typically include or form non-volatile waxes or polymers. During use in a dispenser having a conventional ceramic or sawdust wick, these waxes form sludges that can clog the wick. Thus, ceramic and sawdust wick systems do not allow effective utilization of insecticidal formulas that contain natural pyrethrins and terpenes.
One proposed solution to the problems associated with the clogging of wicks by non-volatiles formed by oxidation of pyrethrum insecticides has been the use of antioxidants in the liquid formulation. See, for example, U.S. Pat. No. 4,968,487. However, in spite of antioxidants, some cross-linking of the isoprene units in insecticidal liquid formulations occurs, forming non-volatile components that degrade the wicking performance of traditional wicks. The performance losses in these systems can be attributed to clogging of the small pores and the effects of the high tortuosity (twists and turns) in conventional wick materials.
Certain disadvantages are also seen with devices that use a heat source to promote the release of a volatile material from a solid or stiff porous mat impregnated with a volatile material or of a volatile material placed in a pan-like metal structure. One problem with the metal pan-like structures is that typical heaters can cause a volatile material to be exposed to too much heat. This can cause the volatile to be used up too fast or be deteriorated or destroyed through thermal degradation. Fibrous mats have similar but somewhat lesser problems in this regard.
Dispensing devices using heated mats also have problems with respect to the mats being exposed to differing temperatures across a heater surface. Existing heaters often have hotter regions at certain points along their heater surface, typically generally along a center line. The mats therefore can have uneven and incomplete vaporization. Uneven mat heating can cause the overall rate at which insect control volatile active ingredients are delivered to drop below the insect controlling rate for the active while considerable quantities of the active remain in poorly heated portions of the mat. Indeed, even aside from the issue of uneven heating, mats tend not to deliver a linear release of volatile materials. Commonly, the initial volatile delivery rate is comparatively high, as active accessibly near the surface of the mat quickly volatilizes. Once that accessible active is gone, migration of active from the interior of the mat to the surface can be slow, at least in part because of small, entrapping pores and very convoluted flow paths. The delivery rate decreases until the mat no longer delivers active at an insect controlling rate and must be replaced. It is not uncommon that conventional insecticide mats for use with conventional heaters have released only about 60% of their load of volatile material before the delivery rate falls below an effective, insect controlling delivery rate. This results in waste and therefore in increased cost for such mat products.
Another design consideration for insect control mats is that existing heaters often only accept slab-like inserts having a small cross-sectional shape, necessary to fit into a small heater loading port or opening. Thus, any mat design preferably takes into account size restrictions imposed by existing heaters. Yet another critical design consideration in this type of device is cost. Mats of this type are often used in countries that have very modest average annual incomes. To have much practical application in those countries, the mats must be inexpensive.
Thus, there is a need for an improved liquid-delivery wick that can be used in a dispensing device that uses a heat source or other active means to promote the release of a volatile material from the wick. Furthermore, there is a need for an improved porous mat that can be used in a dispensing device that uses a heat source or other active means to promote the release of a volatile material from the mat. In particular, there is a need for wicks and porous mats that provide improved efficacy (including but not limited to non-clogging, linear, and complete volatile release) and low cost, together with compatibility with existing dispensing devices.